The present invention relates to a method for transmitting video signals with the aid of differential pulse code modulation (DPCM) and a controlled quantizer, in which the system is switched between different quantizing characteristics.
For the economical transmission of video signals, such as, for example, television signals, various types of DPCM method are already in use. The basic block circuit diagram for such a DPCM system is shown in FIG. 1. An analog video signal of a limited bandwidth from a picture source, which may be a camera, transparency scanner, film scanner or the like, is scanned in an analog-digital converter and each scanned value is linearly quantized at, for example K=256 levels, to produce a digital signal x containing an 8 bit word per picture element (pel). A digital prediction value x, generated by a predictor P, is subtracted from the resulting digital signal x to form a difference signal e which is quantized in a quantizer Q which has K' quantizing output levels, where K'&lt;K to produce a quantized difference signal e.sub.q. This signal e.sub.q is added to x to form a quantized video signal x.sub.q. It is this signal x.sub.q which serves as the input signal to predictor P. The quantized signal e.sub.q is coded in a coder C into constant length code words and is transmitted over the transmission channel.
At the receiver, the received signal is decoded in a decoder D and the recovered difference signal e.sub.q is added to prediction value x in order to regenerate the quantized signal x.sub.q. When the transmission is perfect, the result of the addition x.sub.q corresponds to the original value x except for the quantizing error e-e.sub.q, which is the source of quantization noise. At the same time x.sub.q is fed to a predictor P which is of identical design to the predictor P in the transmitter and which regenerates the prediction value x. The regenerated digital signal x.sub.q is reconverted in a digital/analog converter to an analog signal which can then be supplied to a monitor.
DPCM methods of this type, even if they employ quantization which utilizes 4 bits per picture element (pel), permitting 16 quantization output levels, still produce visible quantizing errors, such as "edge busyness" and "overload", even if complicated two-dimensional predictors are employed, such as disclosed by D. J. Connor, R. F. W. Pease and W. G. Scholes in "Television Coding Using Two Dimensional Spatical Prediction", Bell System Technical Journal, Vol. 50 (1971), at pages 1049-1061.
In order to reduce the visibility of such errors in DPCM systems, methods have been proposed which employ switchable quantizing characteristics, as disclosed by H. G. Musmann in German Patent Application No. P 21, 31, 083.8 of June 23rd, 1971.
FIG. 2 shows a block circuit diagram of such a system which can be switched between n quantizing characteristics. Systems of this type are disclosed by Th. Kummerow, in "Ein DPCM System mit zweidimensionalem Pradiktor und gesteuertem Quantisierer" [A DPCM System With Two-Dimensional Predictor And Controlled Quantizer], NTG-Fachtagung Signalverarbeitung [NTG special conference on signal processing], April 4th to 6th, 1973 at Erlangen, Conference Report, pages 425-439. In this system, quantizer Q of FIG. 1 is replaced by a plurality of quantizers Q.sub.1 . . . Q.sub.i . . . Q.sub.n, and coder C and decoder D are each replaced by a plurality of coders C.sub.1 . . . C.sub.i . . . C.sub.n and decoders D.sub.1 . . . D.sub.i . . . D.sub.n, respectively. Each quantizer corresponds to a respective coder and decoder. A control logic S at the transmitter decides, on the basis of the previously transmitted signal value x.sub.q, which quantizer and coder from the set of quantizers and coders is to be used for each scanned value to be quantized. An identical logic S at the receiver selects the corresponding decoder.
The attainable picture quality depends to a great extent on the selection of the quantizing characteristics and of the control criterion. In particular, the reduction of quantizing noise which is intended to be produced with a controlled quantizer can also be adversely influenced when switching of the quantizing characteristics is effected as a result of noise in the picture source. These switchings may produce visible interference if, for example, in a sequence of television images, different quantizing characteristics are used to quantize a given picture element of a still picture, and thus the picture element is reproduced with quantizing noise which changes in time.